The present invention generally relates to wavelength-division multiplexing and optical communications and, more particularly, to a laser which can be used in such systems.
Wave-division multiplexing is a desirable way to increase the capacity of existing and future optical fiber lines, because it uses the vast frequency domain available in an optical fiber by assigning different wavelengths to different channels. To make such a system even more flexible, it would be desirable to provide lasers which can be tuned to the wavelengths of the different channels, instead of having one laser with a fixed wavelength for each channel. Although continuous tuning over the entire range of wavelengths may not be necessary, it would nonetheless be desirable to have a continuous tuning range around each channel so that the wavelengths can be precisely adjusted to fit the channel.
Excluding external cavity lasers, which, for some types of applications are not very practical, there are two basic categories of tunable semiconductor lasers. The first category includes those lasers which use tunable Bragg gratings and the second category includes those lasers which use an interferometric principle. This first category includes, for example, two and three section Distributed Bragg Reflectors (DBR) lasers, multisection Distributed Feedback (DFB) lasers and sampled-grating DBR lasers. The second category includes, for example, the C.sup.3 laser and the Y-junction laser. Each of these types of lasers has different problems associated with its use in optical communication systems. For example, the multi-section DBR lasers and DFB lasers have limited tuning ranges. The C.sup.3 laser suffers from poor reproducibility and has complex control considerations, while the Y-junction lasers also suffer from control problems.
Another solution is to provide several lasers, each of which lases at a different wavelength, and to then combine their output to produce an optical signal including wavelengths of the different channels. This solution, however, is problematic in that it is relatively expensive since drive electronics are needed for each laser, combining the outputs from lasers with low losses can be difficult and the size of such devices is relatively large.
The tuning range of a conventional DBR laser is limited by the tuning range of a single Bragg grating, i.e., up to a maximum of ten to fifteen nanometers. The so-called sampled grating DBR lasers avoid this limitation by modulating the gratings to generate two combs of sidebands. By aligning a given sideband from one grating with a sideband from the other grating, one can thus select the lasing wavelength. However, obtaining all the desired channel wavelengths may be difficult because the tuning is not continuous and proceeds by jumps. Making these jumps coincide with the channel spacing is a difficult design and fabrication problem. Also, the relation between the control currents and the output wavelength is in general not monotonic (the wavelength may jump back and forth when the currents are increased) which makes control complicated.
Thus, it would be desirable to provide, for example, a DBR laser with an extended tuning range to overcome the shortcomings of conventional lasers in, for example, optical communication applications.